For such household electrical appliances, it is known to monitor the state of charge (current capacity) of the secondary battery in order to indicate to the user the remaining run time of the battery-driven appliance well in advance to give a chance to charge the secondary battery before the appliance stops working. Further, it is known to have a battery control management in order to avoid damage to the secondary battery, e.g. by discharging of the secondary battery below a lower capacity limit.
In this context, the U.S. Pat. No. 6,346,795 B2 describes a discharge control circuit that securely prevents an over discharge of a battery. The control circuit includes a discharge control switch connected to the battery for cutting off discharge current of the battery in response to a discharge stop signal. The discharge stop signal is created when a voltage of at least one cell of the secondary battery reaches a lower limit.
However, lithium-ion secondary batteries often used in household electrical appliances are aging not only with an increasing number of charging and discharging cycles, high or low experienced temperatures, high charging or discharging currents, over charging or over discharging in particular, but also if the state of capacity of the secondary battery during a storage of the household electrical appliance is too high. These phenomena are well known and due to the high cell voltage of the fully charge lithium-ion secondary battery leading to a faster decomposing of the electrolyte compared to a lower charge status (current capacity) of the battery. Decomposition of the electrolyte leads to a rising inner resistance of the secondary battery and a lowering of the maximum capacity. This leads to a shorter possible use time after one charging.
In order to avoid that the secondary battery is already aged when the household electrical appliance is delivered to a customer before its first customer use, the secondary batteries of household electrical appliances are brought into a defined (lower) state of charge with respect to the maximum state of charge of the secondary batteries. To this aim, electrical household appliances are brought into a defined capacity state (state of charge of the secondary battery) by controlling an externally controlled charging and/or discharging process with presumably suited charging or discharging currents to adjust the capacity state of secondary battery as desired. Then, an energy saving stand-by mode of the household electrical appliance is activated. This adjustment of the current capacity of the second battery of the household electrical appliance is time consuming and requires a round-table production for enabling a cyclic interaction towards the end of the manufacturing process in order to timely start and stop charging and discharging processes. At the user's premises, no adjustment of the battery capacity to an optimum storage capacity is performed. Accordingly, the secondary battery might age stronger than necessary.
It is accordingly an object of the present invention to propose a method and a respective household electrical appliance suited to detect a situation of storage and to adjust the state of charge (current capacity) of the secondary battery of the household electrical appliance such that aging of the secondary battery is minimized during longer storage of the household electrical appliance without use.